22-Elec-A4 Digital Systems and Computers · May 2014

Question 6 of 6: Computer System Organisation and the CPU Register Set

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. National Exams, May 2014 — 07-Elec-A4 Digital Systems & Computers. Three hours, closed book (one approved Casio or Sharp calculator). Six questions are printed; any five constitute a complete exam and every question is worth 12 marks, with the per-part split given in the page-1 marking scheme (Q1 is 3+3+3+3; Q2 is 6+3+3; Q3 is 6+6; Q4 is 3+3+6; Q5 and Q6 are 4+4+4). A flip-flop excitation table for the RS/JK/T/D types and a table of 22 basic Boolean identities are supplied on the last page. All six questions are solved below, because this set is a study resource rather than a timed sitting.

Reference texts.

M. M. Mano and M. D. Ciletti, Digital Design: With an Introduction to the Verilog HDL, VHDL, and SystemVerilog, 6th ed. — Boolean algebra, canonical forms and K-maps (Ch. 2–3), combinational MSI decoders and demultiplexers (Ch. 4), synchronous sequential design and flip-flop excitation (Ch. 5), registers and counters (Ch. 6).
J. F. Wakerly, Digital Design: Principles and Practices, 5th ed. — minimisation and static timing hazards (Ch. 3–4), counters and shift registers (Ch. 8).
C. Hamacher, Z. Vranesic, S. Zaky and N. Manjikian, Computer Organization and Embedded Systems, 6th ed. — the four essential subsystems and bus structure (Ch. 1–3), serial interfaces and UART framing (§3.5), the processor register set and addressing modes (Ch. 2).
M. A. Mazidi, R. McKinlay and D. Causey, PIC Microcontroller and Embedded Systems — port-driven multiplexed seven-segment displays and LED current limiting (Ch. 12).

Notation used throughout. A bar and a prime both denote complement: \(\overline{A}\) in the mathematics and A′ in the figures, where SVG text cannot carry an overbar. In every K-map the leftmost variable is the most significant bit, so the minterm indices printed in the cells match the question’s own variable ordering.

Question 6: Computer System Organisation and the CPU Register Set (12 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) The four essential components are those without which no stored-program machine can execute at all. Everything else in the list is a peripheral: useful, often indispensable in practice, but attached to the machine rather than part of it.

Part (a) — essential components marked X
Item	Essential?	Why
Mouse	–	Input peripheral
Processor (CPU)	X	Fetches, decodes and executes instructions; contains the ALU and the register set
Printer	–	Output peripheral
Memory	X	Holds the program and its data — the stored-program principle
USB drive	–	Secondary storage, reached through an I/O port
Busses (address, data, control)	X	The interconnect that carries addresses, data and timing between the other three
Keyboard	–	Input peripheral
I/O ports	X	The interface through which every peripheral is reached
Display monitor	–	Output peripheral
Hard drive	–	Secondary storage, reached through an I/O port

(b) A general-purpose microprocessor is a CPU and essentially nothing else: it brings the register set, the ALU and the control unit onto one chip, and everything else — memory, timers, serial ports, converters — is external and reached over exposed address, data and control buses. That makes the device flexible and fast at the cost of pin count, board area and power, and it means a working system needs several chips. A microcontroller is a complete computer on one chip: a usually simpler CPU core plus program memory (Flash or ROM), data memory (RAM), and a fixed set of on-chip peripherals — parallel ports, timers, a UART, analogue-to-digital converters, PWM outputs and an interrupt controller. Its buses are internal and often invisible outside the package; its pins are peripheral pins, like the \(PB_7\)–\(PB_0\) port used in Question 5.

The consequences follow from that structural difference. Microprocessors dominate where raw throughput and expandable memory matter (workstations, servers), and are optimised for average-case speed with deep pipelines and multi-level caches. Microcontrollers dominate embedded control, where the priorities are cost, low power, small physical size, quick and deterministic interrupt response, and sufficient integration to run a product from one chip. Microcontrollers are also commonly Harvard-architecture with separate program and data memories, whereas general-purpose microprocessors are usually von Neumann at the architectural level. Word sizes differ accordingly: 8 and 16 bits remain entirely normal in microcontrollers, while general-purpose parts are 32 or 64 bits.

(c) Each description names a standard register by its function.

Part (c) — register identification
Description	Register	Comment
Address of the next instruction to be executed	Program counter (PC), also called the instruction pointer	Incremented automatically during each fetch; a branch or call overwrites it
Next available location at the top of the stack	Stack pointer (SP)	Adjusted by PUSH/POP and by subroutine call and return
Pointing to an array or list of data values in memory	Index register (or base/pointer register, e.g. X, Y, IX)	Supports indexed and base-plus-offset addressing modes for array traversal
Information used at decision-making points	Condition-code register (status register, flag register)	Holds the Z, N/S, C and V flags set by the ALU and tested by conditional branches

Question 6 — final results
Part	Answer
(a)	Processor (CPU), Memory, Busses (address/data/control), I/O ports
(b)	Microprocessor = CPU only, external memory and peripherals, exposed buses, high throughput; microcontroller = CPU + memory + peripherals on one chip, internal buses, optimised for cost, power and deterministic embedded control
(c)	Program counter; stack pointer; index (base/pointer) register; condition-code (status/flag) register

Topic. The four subsystems of a stored-program computer, microprocessor versus microcontroller, and the programmer-visible register set.

Key relations. The fetch–decode–execute cycle: the PC supplies an address on the address bus, memory returns an instruction on the data bus, the control unit decodes it, operands are fetched, the ALU computes and sets the condition codes, and the PC advances. Stack discipline: a CALL pushes the return address and adjusts SP; a RET pops it back.

Why this works. The list in part (a) is a test of whether the candidate separates architecture from configuration. A machine with no keyboard and no screen — the microcontroller inside a thermostat, for instance — is still a complete computer, because it retains a processor, memory, an interconnect and a means of reaching the outside world. Remove any one of those four and no program can run. The register questions in part (c) test the same distinction at a lower level: the PC, SP, index register and condition codes are not general storage but architectural state with defined roles in the instruction cycle, which is why they survive across otherwise very different instruction-set architectures.

Common pitfall. Marking the hard drive as essential by confusing storage with memory: secondary storage is reached through an I/O port and holds nothing the CPU can execute directly. In part (c), naming the accumulator for the decision-making register — the accumulator holds the result, whereas the flags that conditional branches actually test live in the condition-code register.

Source. Hamacher, Vranesic, Zaky & Manjikian, Computer Organization and Embedded Systems 6th ed., Ch. 1 (basic structure), §2.4 (addressing modes and index registers), §2.9 (stacks and subroutines).

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